Liquid crystal display (LCD) devices are well known and are useful in a number of applications where light weight, low power and a flat panel displays are desired. Typically, these devices comprise a pair of sheet-like, glass substrate elements, or "half-cells," overlying one another with liquid crystal material confined between the glass substrates. The substrates are sealed at their periphery with a sealant to form the cell or device. Transparent electrodes are generally applied to the interior surface of the substrates to allow the application of an electric field at various points on the substrates thereby forming addressable pixel areas on the display. Examples of useful liquid crystal materials are twisted nematic, super twisted nematic and ferroelectric liquid crystal mixtures.
It is desirable to manufacture large area displays of relatively light weight for use in portable devices such as computers, overhead projectors and the like. Certain organic, polymeric substrates are much lighter than glass and are therefore preferred for use over glass in large area, lightweight displays. However, these substrates tend to be more flexible than glass and must be separated by a dense population of spacers to maintain uniform separation between the closely spaced half cells forming the LCD device. This problem is even more severe with surface stabilized ferroelectric liquid crystal displays which require a nominal 2 .mu.m spacing controlled to within 0.1 .mu.m for good results to produce a uniform electric field at low voltages and show uniform contrast across the entire display area. This uniform spacing is required to provide precise control of the shallow cavity containing the liquid crystal material.
Means for achieving the required spacing uniformity include using either precisely dimensioned, short-length polymeric fibers or spheres as in U.S. Pat. No 4,501,471 or spacing members made of photoresist material bonded to the substrate as in U.S. Pat. No. 4,720,173. Each of these methods has deficiencies. Fiber and spheroidal spacing particles are not easily placed uniformly on the substrate to maintain even spacing over the entire area and fibers may overlap to increase the spacer height. Moreover, when the device flexes or is otherwise physically stressed, the spacers may shift or migrate to cause starved areas in the display cell. Bonded structural members must be precisely positioned on each substrate with exactly the same height, a feat that is difficult given the dimensions and tolerances required for effective liquid crystal displays. If members have different chemical composition from the substrate, differential thermal expansion may occur, which causes possible fracture of the bond at the interface and shifting of the spacing member.
Many of these deficiencies were addressed by imparting polymeric substrates with a microstructure comprising uniform height ridges to maintain uniform cell gaps as disclosed in U.S. Pat. No. 5,268,782 (Wenz), which is incorporated herein by reference. The microstructured substrate disclosed by Wenz provided spacers that would not shift or fracture because the microstructure was physically and chemically integral with the substrate. Therefore, no bonding was required between the spacers and the substrate with which they were integral. In addition, the fabrication of the microstructured substrate could be controlled well within a 0.1 .mu.m or less tolerance even over large areas (tens and hundreds of square centimeters), making it possible to construct large area, light weight displays while preserving uniform contrast across the display.
One primary difficulty with the Wenz approach is that all currently available thermoplastic polymer materials suitable for the formation of the microstructured ridges and having the desired optical properties tend to be soluble in or to absorb either the liquid crystal mixtures or alignment layer solvents during processing, operation, and storage of the devices. Such reactions with the substrate polymer adversely affect the optical properties of the substrate, which may cause problems ranging from aberrations in the LCD to failure of the device.
To prevent interactions between the LCD material and the spacing members, the microstructured surface may be coated with a thin layer (about 500 to 2000 Angstrom) of silicon dioxide by vacuum deposition prior to contact with the liquid crystal or alignment layer materials. While the silicon dioxide layer provides an adequate barrier as deposited, formation of a complete LCD requires exposure of the substrate to elevated temperatures during, for example, alignment layer processing, storage, and post-fill annealing. Exposure to these elevated temperatures causes the silicon dioxide coating to fracture, particularly at high stress points, due to the large difference in coefficients of thermal expansion between the silicon dioxide coating and the polymer substrate. The fractures in the silicon dioxide layer provide areas of contact between the liquid crystal and the substrate. This may result in absorption of the liquid crystal by the substrate at these fracture points, causing local swelling of the substrate. The local swelling leads to failure of the display due to the uncontrolled optical retardation and disruption of the uniform spacing gap caused by the absorbed liquid crystal in the substrate.
Polymers that are impervious to liquid crystal mixtures and to alignment layer solvents may also be applied to the spacer members to form a barrier. These polymers can be chosen or formulated to have coefficients of thermal expansion that closely match that of the substrate polymer. Polymer coatings may be applied over the microstructured substrates disclosed by Wenz using methods such as coating and subsequent cross-linking of a thin liquid resin, or evaporation coating of a polymer layer, over the existing microstructure. However, such polymer coating methods are inadequate to simultaneously provide the required protection of the substrate and the required level of cell gap uniformity between the microstructured ridges. Thin coatings (&lt;0.1 .mu.m) do not provide an adequate barrier, whereas thicker coatings (&gt;0.1 .mu.m) do not replicate the microstructure within tolerances and thus cause display non-uniformities.
Another shortcoming with the Wenz approach is that it is not transferable to glass substrates or to substrates other than those that can be imparted with the microstructure. For such substrates, less reliable spacer means, such as fibers, glass beads, and photoresist ribs would need to be employed.
A major driving force for the advancement of electronic display technology is the ability to provide larger displays having higher resolution. Such advances cannot be made without the development of substrate materials and materials combinations that may be adapted to large area display applications and allow precise control of sub-micrometer dimensions.